The invention relates to new, low viscosity radiation-hardenable urethane acrylates that harden to form coatings having improved abrasion resistance, as well as their use as coating compositions, in particular for flooring materials.
Radiation-hardenable coating compositions based on reaction products of hydroxyfunctional esters of (meth)acrylic acid and diisocyanates are termed urethane acrylates and are known for example from P.K.T. Oldring (ed.), Chemistry and Technology of UV and EB Formulations for Coatings, Inks and Paints, Vol. 2, 1991, SITA Technology, London pp. 73-123. They are frequently used to coat parquet and other materials used as floorings. Such coating compositions generally have a high dynamic viscosity of typically more than 10000 mPa.s (at 23xc2x0 C.) and are therefore diluted with low molecular weight acrylic acid esters (reactive diluents), and are applied together with the addition of photoinitiators and optionally additives by various methods, such as for example roller application, to substrates to be coated and are then hardened under the action of UV radiation. Due to the dilution with reactive diluents the urethane acrylates frequently suffer a deterioration in important properties such as abrasion toughness and viscoelasticity. It is therefore expedient to use particularly low viscosity urethane acrylates in order to keep the amount of reactive diluents low. Readily available starting products for the production of urethane acrylates include hydroxyfunctional esters of (meth)acrylic acid, in most cases 2-hydroxyethyl acrylate or methacrylate as well as 2-hydroxypropyl acrylate or methacrylate. The resulting urethane acrylates are however highly viscous when undiluted (see examples from EP-A 168173).
EP-A 53749 discloses the production of low viscosity urethane acrylates based on diisocyanates or polyisocyanates, hydroxyfunctional acrylic acid esters of 3-fold to 4.5-fold oxalkylated trimethylolpropane and optionally a hydroxyalkyl acrylate. In the cited application, comparison tests show that even with a degree of oxethylation of 7 the corresponding products are insufficiently resistant to polar solvents.
It is an object of the present invention to provide low viscosity urethane acrylates that are resistant to solvents and that have improved abrasion resistance.
It has now been found that urethane acrylates based on diisocyanates or polyisocyanates and hydroxyfunctional (meth)acrylic acid esters of mixtures of high-oxalkylated and low-oxalkylated polyols have a low viscosity, are resistant to solvents, and are particularly abrasion-resistant. This was surprising since it was not expected that there would be a significant difference between urethane acrylates containing for example a 7-fold ethoxylated triol, and urethane acrylates containing a mixture of higher and lower ethoxylated triols that corresponds on average to a 7-fold ethoxylated triol.
The invention relates to a urethane acrylate prepared from a diisocyanate and/or polyisocyanate and a hydroxyfunctional partial ester wherein the ester is the reaction product of acrylic acid and/or methacrylic acid with a mixture of oxalkylated polyols with three or more hydroxyl groups, wherein the mixture of the oxalkylated polyols contains 25 to 75 mole % of a polyol having a degree of oxalkylation between 3 and 5 and 75 to 25 mole % of a polyol having a degree of oxalkylation between 8 and 25.
The invention also relates to a process for making these urethane acrylates in a two-stage production process by partially esterifying in a first stage an oxalkylated polyol with acrylic acid and/or methacrylic acid and in a second stage reacting the resulting product with a diisocyanate and/or a polyisocyanate.
The invention also relates to a material coated with a coating containing this urethane acrylate.